AS NOTED IN THE PREFACE, both my article on Iggeret ha-Shemad, which I wrote as an undergraduate, and that on martyrdom in Ashkenaz, composed a decade or so ago, employ the same criterion of ‘measurable deflection’ to discern whether something extraneous to the halakhic system is impinging upon the thought of a halakhist. That is to say, in order to avoid a simplistic sociology of law that attributes, with little ado, legal conclusions to personal inclinations or some social or economic need, one must be able to point to some uncharacteristic flaw, or, at the very least, some measurable swerve in the line of argument that would indicate the operation of a force deflecting the author's analysis from its normal course and leading him to unexpected conclusions. Unsurprisingly, this yardstick plays a significant role in other studies of mine published in the intervening years, such as ‘Can Halakhic Texts Talk History?’ and ‘Pawnbroking: A Study in Ribbit and of the Halakhah in Exile’.

There has been some misunderstanding concerning the nature of this tool, and a word or two may be in place. ‘Measurable deflection’ is not a jurisprudential criterion, as some have thought, but an evidentiary one. That is to say, whether law develops immanently, and the judge's role is that of an outsider simply ascertaining what the normative texts say on the problem—as the judicial formalists believe—or whether the judge is consciously or unconsciously a social or religious engineer interpreting the pliable normative texts in such a manner that the results conform to his Weltanschauung—as the judicial realists contend—has been a long-standing question in jurisprudence. However, the issue between the formalists and the realists is the nature of the predominant mode of adjudication; each readily admits that ‘predominant’ does not mean universal and that there are any number of exceptions to the system's usual manner of operation. Measurable deflection addresses the question, When does a historian have the right to say that a specific decision of a specific judge was shaped by external considerations? How judges generally act is a jurisprudential question; why Judge X ruled as he did in Case Y is a historical one. Even in law, ‘general propositions do not decide specific cases’, as Justice Holmes pointed out long ago.